C++ developers,
Over 90% of the CPU time required to build the tree is spent compiling
or linking C/C++. So, anything we can do to make that faster will make
the overall build faster.
I put together a quick patch [1] to make it rather simple to extract
compiler resource usage and very basic code metrics during builds.
Simply apply that patch and build with `mach build --profile-compiler`
and your machine will produce all kinds of potentially interesting
measurements. They will be stuffed into objdir/.mozbuild/compilerprof/.
If you don't feel like waiting (it will take about 5x longer than a
regular build because it performs separate preprocessing, ast, and
codegen compiler invocations 3 times each), grab an archive of an OS X
build I just performed from [2] and extract it to objdir/.mozbuild/.
I put together an extremely simple `mach compiler-analyze` command to
sift through the results. e.g.
$ mach compiler-analyze preprocessor-relevant-lines
$ mach compiler-analyze codegen-sizes
$ mach compiler-analyze codegen-total-times
Just run `mach help compiler-analyze` to see the full list of what can
be printed. Or, write your own code to analyze the produced JSON files.
I'm sure people who love getting down and dirty with C++ will be
interested in this data. I have no doubt that are compiler time and code
size wins waiting to be discovered through this data. We may even
uncover a perf issue or two. Who knows.
Here are some questions I have after casually looking at the data:
* The mean ratio of .o size to lines from preprocessor is 16.35
bytes/line. Why do 38/4916 (0.8%) files have a ratio over 100? Why are a
lot of these in js/ and gfx?
* What's in the 150 files that have 100k+ lines after preprocessing that
makes them so large?
* Why does lots of js/'s source code gravitate towards the "bad" extreme
for most of the metrics (code size, compiler time, preprocessor size)?
Disclaimer: This patch is currently hacked together. If there is an
interest in getting this checked into the tree, I can clean it up and do
it. Just file a bug in Core :: Build Config and I'll make it happen when
I have time. Or, if an interested party wants to champion getting it
landed, I'll happily hand it off :)
[1] http://hg.gregoryszorc.com/gecko-collab/rev/741f3074e313
[2] https://people.mozilla.org/~gszorc/compiler_profiles.tar.bz2
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